Studies on a circulating anticoagulant inhibiting factor XI in a patient with congenital deficiency and carcinoma of the prostate

An inhibitor of plasma thromboplastin antecedent (PTA, factor XI), measured in coagulant and radioimmunoassays, was detected in a 60-year-old man with carcinoma of the prostate who had no evidence of a bleeding tendency. Family studies indicated that the patient was either a homozygote or a heterozygote for hereditary factor XI deficiency. In contrast to earlier described patients with factor XI deficiency in whom inhibitors were detected, the patient was unaware of having been transfused with blood or blood products at any time before the discovery of the inhibitor. The inhibitor of factor XI in the patient's plasma appeared to be predominantly in the IgG4 fraction and to be directed at a locus on the factor XI molecule other than the active site; it did not block the amidolytic properties of activated factor XI (XIa). Rather, it appeared to block adsorption of factor XI to negatively charged surfaces. The inhibitor interfered with measurement of other components of the intrinsic pathway of thrombin formation, perhaps explaining the low titres of other coagulation factors of the intrinsic system reported in patients with strong inhibitors directed against factor XI.     

Effect of heparin on the inactivation rate of human factor XIa by antithrombin-III

Factor XIa catalyzes an important reaction in the early phase of blood coagulation by converting factor IX to an active enzyme (factor IXa). Although antithrombin-III, an inhibitor of factor XIa, normally accounts for only one-sixth of the plasma inhibitory activity against factor XIa, its effectiveness has been reported to be enhanced by heparin. We have reinvestigated the ability of heparin to potentiate factor XIa inhibition by both purified antithrombin-III and plasma using synthetic tripeptide amide substrates as well as a coagulant assay. No increase in the inactivation rate of factor XIa amidolytic activity by purified antithrombin-III was observed in the presence of therapeutic heparin concentrations (1 U/ml), although inhibition of the amidolytic activity of thrombin by purified antithrombin-III was enhanced at least 20-fold by the same concentration of heparin. Furthermore, despite the ability of heparin (1 U/ml) to increase the inactivation rate of thrombin by plasma, no acceleration of the rate of inhibition of factor XIa by plasma was observed. Similar results were found when the inhibition of factor XIa was monitored with a coagulant assay after first removing the heparin. Only at heparin concentrations of 5 and 10 U/ml, was a 2- and 4-fold increase in the inactivation rate of factor XIa by purified antithrombin III observed. Therefore, in both purified systems as well as plasma, heparin, at concentrations observed in clinical practice, does not accelerate the inactivation rate of human factor XIa by antithrombin-III.     

Factor XI antigen and activity in human platelets

Washed platelets, contaminated with less than 0.20% plasma factor XI, were examined for the presence of factor XI antigen and activity. These platelets contained a factor-XI-like coagulant activity (0.67 +/- 0.11 U/10(11) platelets) that remained constant after successive washes. By means of indirect immunofluorescence, a monospecific antibody to factor XI showed specific staining of both normal platelets and platelets from patients deficient in plasma factor XI. Radiolabeled Triton extracts of washed platelets and labeled purified factor XI solutions were analyzed for factor XI antigen by Staph A immunoprecipitation analysis using antibody to purified plasma factor XI followed by SDS gel electrophoresis. On unreduced gels, the platelet material ran as a single band having an apparent molecular weight of 220,000 daltons, whereas purified plasma factor XI gave a single band at 160,000 daltons. On reduced gels, the platelet material analyzed as a single band at 52,000 daltons, whereas purified factor XI gave a single band of 80,000 daltons. Analysis of a partially purified factor XI preparation from platelets by immunoelectrophoresis revealed that the platelet preparation displayed a slightly lower cathodal electrophoretic mobility at pH 8.6 than did plasma factor XI and yet appeared to possess complete antigenic identity with plasma factor XI. These results indicate that platelets possess a form of factor XI that exists as a disulfide-linked 52,000-dalton tetramer in contrast to the plasma form that circulates as a 80,000-dalton disulfide-linked dimer.     

Assay of prekallikrein in human plasma: comparison of amidolytic, esterolytic, coagulation, and immunochemical assays

Using the substrate H-D-Pro-Phe-Arg-p-nitroanilide-HCl, an amidolytic assay was designed to measure prekallikrein in plasma. At a substrate concentration of 1 mM (Km = 0.2 mM), the amidolysis of purified kallikrein at 1 coagulant unit/ml was observed to be 2.47 mumole/min/ml. Conditions for plasma prekallikrein activation were optimized to approach complete activation when compared to the amidolytic activity of the purified plasma kallikrein. Plasma treated with chloroform to destroy inhibitors of kallikrein was activated with dilute kaolin (final concentration 1 mg/ml) for 1 min at 25 degrees C. Activated plasma prekallikrein had 78% (1.92 mumole/min/ml) of activity of purified kallikrein at plasma concentration. Comparison of this amidolytic assay with immunochemical, esterolytic, and coagulant assays of three subject populations (normals, women on birth control pills, and patients with hepatocellular disease) showed good correlation both in normals and in the patient groups between the amidolytic and esterolytic assays (r = 0.89). Each enzymatic assay correlated with the immunochemical assay (r = 0.72, r = 0.68, respectively). However, comparison of each of these assays with the coagulant assay showed no significant correlation due to the large inherent error of the latter assay. This standardized plasma prekallikrein amidolytic assay should facilitate studies of plasma prekallikrein concentration in physiologic and pathologic conditions and help identify activation of the contact phase of coagulation in disease states.     

Pseudo-factor-XI deficiency: effect of an inhibitor of factor XI adsorption to surface

Recently we have described a normal plasma activity that modulates contact activation by inhibiting adsorption of factor XI to activating surfaces. Here we report the first identified case in which a patient has abnormal clotting tests due to an excess of a similar activity. The patient's plasma had a prolonged partial thromboplastin time and low apparent factor XI assay. His plasma prolonged the partial thromboplastin time of normal plasma and partially neutralized normal factor XI activity in vivo and in vitro. Analysis in dilute plasma revealed normal amounts of factor XI activity and antigen. Factor XI adsorption from plasma to activating surfaces was tested by adding a small amount of 125I-labeled purified factor XI to plasma, exposing the mixture to a glass tube or kaolin, and determining the amount of factor XI adsorbed to the surface. Whereas normal plasma and plasmas deficient in factor XII, factor XI, or Fletcher factor yielded about 4% adsorption to glass, factor XI adsorption from patient's plasma was less than 1%, indicating the presence of an adsorption inhibitor. This inhibitor did not affect factor XI activation or the activity of preformed factor XIa. It was not adsorbed by AI(OH)3 and was present in serum and the macroglobulin peak on gel filtration of the plasma through Sephadex G-200. The patient's history does not allow a definitive conclusion as to whether this inhibitor was associated with abnormal bleeding.     

Quantitation of coagulant antigens and inhibition of activated protein C in combined factor V VIII deficiency

Inhibition of activated human protein C was assessed in an amidolytic assay system using normal human plasma and samples from patients with hereditary coagulation abnormalities. In eight experiments normal plasma inhibited 63.5% (+/- 15.6%) protein C activity. Plasma from patients with haemophilia A or isolated factor V deficiency gave results which were not significantly different from normal. However, plasma from patients with combined factor V and factor VIII deficiencies inhibited an average of 24.5% (+/- 13.6%) of the amidolytic activity (P less than 0.01). Two of these plasma samples failed to inhibit any protein C activity. The relationship between the level of inhibitor and those of factor V and factor VII coagulant antigens (VCAg and VIIICAg) in the combined defect was investigated. There was no significant correlation between the level of inhibitor and any of the coagulation immunoassays on these stored samples but there was significant correlation between VCAg and VIIICAg in some assay systems. The levels of VCAg and VII CAg was low in most samples from patients with the combined defect which was in contrast to the results obtained when normal plasma was incubated with activated protein C in vitro. The findings are consistent with the presence of biochemical similarities between factors V and VIIIC molecules, but the role of activated protein C and its inhibitor in hereditary combined factor V/VIII deficiency remains to be firmly established.     

Clotting time analysis of citrated blood samples is strongly affected by the tube used for blood sampling.

Our aim was to establish whether differences in clotting times for recalcified blood and plasma samples might be explained by the use of different blood collection tubes. Samples obtained from different plastic vacuum tubes were recalcified and clotting times determined by free oscillation rheometry. The clotting times for blood collected in Vacutainer (Becton Dickinson, Rutherford, New Jersey, USA) or Vacuette (Greiner Bio-One, Kremsmünster, Austria) tubes decreased with time, with maximal effect after 30 min. Blood from Monovette (Sarstedt, Nümbrecht, Germany) tubes displayed longer clotting times, which did not decrease with time. Clotting times for plasma prepared after 1 h storage in Vacutainer or Vacuette tubes were unaffected by subsequent addition of corn trypsin inhibitor to inhibit factor XIIa, although an antibody against factor XI prolonged the clotting time markedly. In Monovette plasma, both corn trypsin inhibitor and anti-factor XI effectively prolonged the clotting time. When corn trypsin inhibitor or anti-factor XI was added to the tubes before blood collection, but both additions clearly prolonged the clotting times in all types of tubes, even though corn trypsin inhibitor was less effective in whole blood. Antibodies against human tissue factor did not affect the clotting times. The amounts of platelet or leukocyte microparticles in plasma were low and similar in all tubes. This indicates that blood collection in Vacutainer or Vacuette tubes induces a rapid activation of factor XII and factor XI.     

Preparation, Characterization, and Activation of a Highly Purified Factor XI: Evidence that a Hitherto Unrecognized Plasma Activity Participates in the Interaction of Factors XI and XII

S ummary . Highly purified native factor XI has been prepared from normal plasma using a five step purification scheme. Purified factor XI is essentially free of factors II, V, VII, VIII, IX, X, XII, and Fletcher factor. No plasminogen-plasmin could be detected. Purified factor XI decays rapidly but can be stabilized with human serum albumin. Factor XI migrates between the β and γ globulins on starch block electrophoresis. It has an apparent molecular weight on gel filtration of 210 000. Purified factor XI is activated by weak trypsin. No detectable BAEe esterase activity accompanies this activation. Neither factor XII adsorbed to kaolin nor activated factor XII in solution could activate purified factor XI; both reagents activate factor XI in dilute factor XII deficient plasma. Hence, plasma appears to supply a third activity which facilitates the interaction of factors XI and XII. This activity is shown to be distinct from Fletcher factor.     

Comparison of bleeding tendency, factor XI coagulant activity, and factor XI antigen in 25 factor XI-deficient kindreds

The relationship of clinical bleeding tendency and factor XI antigen (XI:Ag) in factor XI deficiency was studied in 78 members of 25 factor XI-deficient kindreds. Factor XI:Ag was measured in a competitive radioimmunoassay, using monospecific, heterologous anti-factor XI antibody. 125I-labeled factor XI, and staphylococcal protein A as the precipitating agent. Deficiency of factor XI clotting activity (XI:C), less than 0.62 U/mL, occurred in 48 individuals, 22 of whom experienced postoperative or posttraumatic bleeding: Their mean factor XI:C was 0.21 +/- 0.04 U/mL (SEM), and factor XI:Ag was 0.23 +/- 0.04 U/mL. The remaining 26 had no clinical bleeding, many despite surgical challenge: Their mean factor XI:C was 0.30 +/- 0.04 U/mL, and factor XI:Ag was 0.34 +/- 0.05 U/mL. In all, 13 kindreds had between 1 and 11 members with bleeding; the other 12 had none with deficient hemostasis. Two heterozygous factor XI-deficient individuals appeared to be positive for cross-reacting material (CRM+). The slope of the regression line for factor XI:C and factor XI:Ag data points in the 78 individuals tested did not differ from control, and all points fell within 95% confidence limits derived from control. In conclusion, bleeding tendency appears to be consistent within a given kindred and is not determined exclusively by factor XI:C or factor XI:Ag levels.     

An autoantibody to human plasma prekallikrein blocks activation of the contact system

. A patient without a history of bleeding or thromboembolism presented with an activated partial thromboplastin time (aPTT) of 55.1 s (normal 24-38 s). Incubation of the patient plasma with an equal volume of normal plasma failed to correct the aPTT. suggesting the presence of an inhibitor. The MRVVT (modified Russell Viper venom time) was normal, and the anti-cardiolipin antibody titres were not elevated, indicating that the presence of a lupus anticoagulant was unlikely. Plasma prekallikrein (PK) measured by a coagulant assay (2 U/dl) was very low, but PK was in the low normal range (65%) when measured by an enzymatic assay (amidolytic) or by an antigenic assay (ELISA). The purified patient IgG reacted with purified PK. the heavy chain, and the 28 kD fragment of the heavy chain, indicating that it contained an autoantibody to PK. The purified IgG did not directly inhibit the amidolytic activity of kallikrein, but it did inhibit the activation of PK to kallikrein by activated factor XII. Activation of the contact system by dextran sulphate, as reflected by the cleavage of HK on a Western blot, was inhibited when the patient IgG was added to pooled normal plasma. The antibody appears to be oligoclonal with IgG1 being most abundant. followed by IgG4. This report appears to be the first of a spontaneously occurring antibody to prekallikrein.     

Identification of a defective factor XI cross-reacting material in a factor XI-deficient patient

A homozygous factor XI-deficient girl, who appeared to be positive for cross-reacting material (CRM+) was studied for clarification. Factor XI antigen (F XI:Ag) was measured by radial immunodiffusion using monospecific, heterologous anti-factor XI antibodies. Factor XI coagulant activity (F XI:C) was determined in a modified activated partial thromboplastin time (APTT) test. The ratio of F XI:C to F XI:Ag was 0.04 for the proposita, as compared with 0.7 to 0.74 in the other family members. In contrast, 12 normal individuals had ratios of F XI:C to F XI:Ag of 1.04 +/- 0.15. F XI esterolytic activity was clearly higher than F XI:C in the proband, but not in her relatives. Immunoblotting studies demonstrated F XI CRM in the patient's plasma. Chromatography on diethylaminoethanol (DEAE)-Sephadex at pH 8.4 led to an almost complete removal of F XI from the plasma. The defective F XI was not bound to a negatively charged kaolin surface due to an abnormal interaction with high-mol-wt kininogen (HMWK).     

Anticoagulant versus amidolytic activity of tissue factor pathway inhibitor in coronary artery disease.

In view of raised levels of endothelial markers in coronary artery disease (CAD), the aim of the present study was to investigate the status of tissue factor pathway inhibitor (TFPI), another endothelium-associated glycoprotein and coagulation protease inhibitor, in CAD. The intravascular pool of TFPI is heterogeneous with respect to assay-dependent activity. While the standard amidolytic assay works well with both full-length and truncated (lipoprotein-associated) TFPI, the anticoagulant assay works better with the former. The anticoagulant activity of TFPI can be estimated using dilute tissue factor (TF) to trigger clotting of plasma. In the present study, recombinant TF diluted 2,000-fold was used to initiate coagulation. A dose-dependent shortening of clotting time of normal plasma pools with polyclonal antibody against the C-terminal but not the N-terminal peptide of TFPI demonstrated the importance of the C-terminal region, and hence that of full-length TFPI, in conferring its anticoagulant activity, corroborating current opinion. As a further confirmation, the C-terminal peptide itself prolonged dilute TF clotting time of normal pooled plasma in a concentration-dependent manner. The amidolytic and anticoagulant activities of TFPI were determined in 20 patients with clinically and angiographically assessed CAD and in 68 asymptomatic controls. The mean +/- SD ages of patients and controls were 54.9 +/- 10.3 and 48.8 +/- 11.6 years, respectively, the difference being statistically significant (P = 0.04). The mean TFPI activity measured by amidolytic assay was comparable for patients and controls (1.2 +/- 0.3 and 1.3 +/- 0.5 U/ml, respectively). However, the dilute TF clotting time was 115 +/- 26 s in patients, against 99 +/- 10 s in controls (P < 0.0001, irrespective of age adjustment). Since none of the patients had received heparin or had coagulation factor deficiency that may interfere with the assay, prolongation of clotting time may be attributed to the presence of TFPI, particularly the full-length form. To verify this inference, 33 extra aliquots left over from 88 samples (62.5%), 21 from controls and 12 from patients, were incubated with 1:10 diluted antibody against the C-terminal peptide of TFPI prior to dilute TF assay. The mean clotting time of both patients and controls decreased, and the between-group difference leveled (90 +/- 10 versus 88 +/- 20 s for controls and patients, respectively; P = 0.841). The mean drop in clotting time was 9% for the controls and 24% for the patients. This illustrates the specificity of dilute TF assay for full-length TFPI and supports the conclusion that relative to lipoprotein-associated TFPI, the proportion of the full-length form was possibly greater in patients with CAD. Contribution of lipoprotein-associated TFPI to the overall anticoagulant activity by its activated factor X-dependent inhibition of activated factor VII-TF complex seems less important considering the similar between-group mean amidolytic activities.     

Changes in high molecular weight kininogen levels during and after cardiopulmonary bypass surgery measured using a chromogenic peptide substrate assay.

High molecular weight kininogen (HK) is a co-factor in the blood-contact activation system. A chromogenic peptide substrate assay for HK (HKcs) has been developed in which test plasmas are mixed with diluted HK-deficient plasma and incubated with a soluble contact system activator that activates prekallikrein and factor XII. Calcium chloride, a synthetic thrombin inhibitor and a chromogenic peptide substrate for activated factor X (FXa) are then added. The FXa generated cleaves the FXa substrate releasing p-nitroanaline, which is measured photometrically. Test plasma HK values were calculated from a standard curve generated using a pooled normal plasma. Acceptable intra-assay and inter-assay precision values were obtained and levels of HK up to 200% were measurable. The assay measured HK in plasmas deficient in factor XII, prekallikrein and factor XI, was not affected by antiphospholipid antibodies and gave an acceptable correlation (r = 0.95) when normal plasmas and mixtures of HK-deficient and normal pooled plasma, calculated to give HK levels of 25 and 50%, were compared using HKcs and a HK one-stage clotting assay. The HKcs was used to measure HK levels in seven patients undergoing cardiopulmonary bypass (CPB). HK levels fell significantly during CPB (P = 0.0014) and were significantly higher (P = 0.016) 6 days after CPB, suggesting that HK may be a positive acute-phase reacting protein.     

How to measure factor VII and factor VII activation

This communication describes the different techniques that can be used to evaluate the activity state of factor VII in plasma samples. At the present time direct methods for quantitation of activated factor VII (factor alpha-VIIa) are not available. Combined methods are therefore used to measure the degree of factor VII activation. These methods can be summarized in the following way: a regular factor VII clotting assay measuring factor VII coagulant activity (factor VIIc) is carried out simultaneously with: (1) a factor VII antigen assay (factor VIIag): (2) a coupled amidolytic factor VII assay (factor VIIam), or (3) a factor VII clotting assay utilizing bovine tissue thromboplastin (VIIbt). The activity state of factor VII can then be calculated from either of the following ratios: f.VIIc/f.VIIag; f.VIIc/f.VIIam, or f.VIIbt/f.VIIc. A study of the potency of a 30-fold activated factor VII to activate factor X in the presence of phospholipids is also included. This experiment demonstrates that even a low factor VII concentration (0.01 U/ml in the coupled amidolytic assay and 0.30 U/ml in the factor VII clotting assay) causes a significant activation of factor X when incubated together in the presence of lipids and calcium ions. Activated factor VII may therefore possess potential thrombotic properties even in the absence of exposed tissue thromboplastin in the blood circulation.     

Nonplasminogen-dependent protease in human plasma.

Equal volumes of plasma and 0.3 M K2HPO4, pH 7.4, were mixed, diluted 20-fold, and adjusted to pH 5.2. After incubation at 37 degrees C for 30 min, the euglobulin percipitate, redissolved in 0.1 M K2HPO4, pH 7.4, developed caseinolytic activity (0.05 CTA U/ml). Na2HPO4 or NaCl of similar ionic strength could replace K2HPO4. The pH optimum of the protease was 6.5, activity falling off sharply below pH 6.0 and above 7.4. The proteolytic activity was inhibited by diisopropylphosphofluoridate and by pancreatic trypsin inhibitor, but was not inhibited by soybean trypsin inhibitor. The activity was not due to plasmin, contact activation, or coagulation factors, since it was fully generated in plasminogen-depleted, factors XII, XI, VII deficient, and prekallikrein-deficient plasmas. Purified Cl-esterase was not caseinolytic in our system. Redissolved euglobulin precipitate prepared from normal plasma without salt addition could serve as starting material for the generation of caseinolytic activity, as could serum, indicating that the Hageman factor cofactor and thrombin are not required. The protease had no detectable procoagulant or fibrinolytic activity.     

Coupled amidolytic assay for factor VII: its use with a clotting assay to determine the activity state of factor VII

A coupled amidolytic assay for factor VII (VII) has been developed that when used with a clotting assay for VII enables detection of activated VII. In the assay, VII in a test material determines generation of factor Xa in a mixture of purified factor X, tissue factor, and calcium; factor Xa is measured with a chromogenic substrate. Factor VII activity in the coupled amidolytic assay (VIIam) correlated well with VII activity in a one-stage clotting assay (VIIc) in 57 healthy subjects, 5 patients with hereditary VII deficiency, and 11 patients with liver disease. Activation of plasma VII by kaolin, clotting, or cold strikingly increased VIIc but not VIIam levels. Thus the ratio VIIc/VIIam (VII activity ratio) is a measure of VII activation. In 27 warfarin-treated patients the mean VII activity ratio was significantly decreased, reflecting a greater decline in VIIc than in VIIam. This probably stems from partially carboxylated VII being able to act during the 3-min incubation of the amidolytic assay but unable to act rapidly enough to affect the clotting assay. Measurement of VIIc/VIIam should enable evaluation of the activity state of VII in thrombotic disorders and in components for transfusion therapy.     

A monoclonal antibody that inhibits activation of human Hageman factor (factor XII)

A monoclonal antibody to human Hageman factor (HF, factor XII) was derived from BALB/c mouse spleen cells fused with NS-1 mouse myeloma cells. This antibody, purified from ascites fluid, reacted with HF to inhibit the activation of HF, purified or in normal pooled plasma, as measured by a coagulation assay. The antibody did not inhibit the coagulant activity of activated HF. The antibody also inhibited the generation of amidolytic activity in HF-ellagic acid mixtures, but failed to inhibit the amidolytic properties of the carboxy-terminal fragment of HF (HFf). Amidolytic activity, absent in an HF-monoclonal antibody mixture, was generated upon treatment with insoluble trypsin. Monoclonal antibody, bound to CNBr Sepharose 4B gel (Pharmacia Fine Chemicals, Piscataway, NJ), reversibly bound HF in plasma or in buffer, without activating it. HF was then eluted with 4 mol/L guanidine HCI. The passage of 125I-labeled HF enzymatically cleaved by trypsin through a column of monoclonal antibody-CNBr Sepharose 4B gel resulted in flow- through of HFf with a molecular weight (mol wt) of 30,000 and HF fragments of mol wt 12,000. Elution with 4 mol/L guanidine HCI yielded several HF fragments (mol wt 80,000, 52,000, and 40,000) but not HFf. These data suggest that the single determinant recognized by the murine monoclonal antibody is not on HFf, but rather on the amino-terminal fragment thought to be involved in the binding activity of HF. The monoclonal anti-HF bound to CNBr-activated Sepharose 4B gel could be used to artificially deplete plasma samples of HF.     

Cabbage seed protease inhibitor: a slow, tight-binding inhibitor of trypsin with activity toward thrombin, activated Stuart factor (factor Xa), activated Hageman factor (factor XIIa), and plasmin

An inhibitor of procoagulant and fibrinolytic enzymes was derived from cabbage seeds by a procedure using acetone precipitation, ion-exchange chromatography, and gel filtration. The cabbage seed inhibitor was a 10-Kd monomeric protein with intrachain disulfide bonds. This preparation prevented clot formation in whole blood and blocked the ability of thrombin to induce clot formation in plasma and to induce platelet aggregation. A number of proteases were inhibited, as demonstrated by using purified enzymes in amidolytic assays. Tight-binding inhibition was observed for activated Stuart factor (factor Xa) and plasmin. Inhibition of thrombin and activated Hageman factor (factor XIIa) was observed with a molar excess of inhibitor. No inhibition was detected for activated plasma thromboplastin antecedent (factor XIa), plasma kallikrein, or C1 esterase. Reaction progress curves for trypsin indicated slow, tight-binding inhibition, with an apparent inhibition constant in the nanomolar range or less. The electrophoretic mobility of trypsin was altered by the inhibitor in nondenaturing polyacrylamide gel electrophoresis (PAGE) but not in sodium dodecyl sulfate (SDS)-PAGE, indicating noncovalent bonding. Only partial reversal of trypsin inhibition could be demonstrated by washing the inhibitor from enzyme immobilized on solid beads. A dot-blot technique with cabbage seed inhibitor was capable of detecting 10 ng nitrocellulose-bound trypsin. The dot-blot technique also appeared capable of detecting plasmin. These findings demonstrated the potential utility of this inhibitor as a probe for detection of tightly bound proteases. In summary, cabbage seed extracts contain an inhibitor with activity toward a broad range of proteases important to hemostasis. To our knowledge, this agent represents the first inhibitor isolated from a plant source that inhibits thrombin.     

Immunologic studies of human coagulation factor XI and its complex with high molecular weight kininogen

Coagulation factor XI was purified from human plasma using ion-exchange chromatography and affinity chromatography on high molecular weight kininogen-Sepharose. A monospecific precipitating antiserum was prepared and used to study factor XI antigen. Factor XI did not migrate during electrophoresis at pH 8.3. High molecular weight kininogen (HMWK), an alpha globulin, reversibly associates with factor XI. Complex formation between HMWK and factor XI was observed under conditions of crossed-immunoelectrophoresis. Using Laurell rocket immunoelectrophoresis, it was shown that the isolated alkylated light chain of kinin-free HMWK formed a complex with factor XI. In contrast to previous studies of prekallikrein, titrations of factor XI with increasing amounts of HMWK did not give a simple titration curve, suggesting that factor XI dissociates from the complex during electrophoresis. Prekallikrein and factor XI were shown to compete for the same HMWK molecules under the conditions of immunoelectrophoresis, and prekallikrein appeared to have a higher affinity for binding to HMWK than factor XI. Quantitative determinations of factor XI antigen in plasma by rocket immunoelectrophoresis were made. The average amount of factor XI measured in plasma samples from 20 normal individuals was 4.5 micrograms/ml (range 3-6). No factor XI antigen was detected in plasma from a patient deficient in factor XI. Normal factor XI antigen levels were detected in 3 different HMWK-deficient plasmas only if the plasmas were reconstituted with purified HMWK (2 U/ml). Addition of HMWK to normal plasma resulted in an increase of the factor XI antigen rocket. At HMWK levels of 2 U/ml, no further increase of the factor XI antigen rocket was observed. Therefore, accurate measurement of factor XI antigen by rocket immunoelectrophoresis is possible only if an excess of HMWK is present.     

Platelets and Initiation of Intrinsic Clotting

S ummary . Comparison of activities in platelet rich and platelet poor plasmas from normal donors and patients deficient in either factor VIII, IX, XI or XII indicates that platelets contain activities which can partially substitute for plasma factors XI and XII. The factor-XI-like activity is expressed in a one-stage activated partial thromboplastin assay and in an intact prothrombin consumption system. The factor-XII-like activity is scarcely detectable in a one-stage assay but markedly enhances the defective prothrombin consumption of factor XII deficient plasma. Intact prothrombin consumption tests with platelet poor plasmas fortified with cephalin show that in the presence of high concentrations of platelet factor 3 activity only trace contact activation is required to promote good prothrombin consumption. The platelet, by supplying both platelet factor 3 and activities bypassing plasma contact activation factors XI and XII, may provide an important route for activating intrinsic clotting.